Pneumatic cylinder for applying tension to riser pipe



R. P. VINCENT Nov. 21, 1967 PNEUMATIC CYLINDER FOR APPLYING TENSION TORISER PIPE 2 Sheets-Sheet 1 Filed Nov. 26, 1965 FIG. I

INVENTOR. RENIC P. VINCENT BY P a! M ATTORNEY R. P. VINCENT Nov. 21,1967 PNEUMATIC CYLINDER FOR APPLYING TENSION TO RISER PIPE Filed Nov.26, 1963 2 Sheets-Sheet 2 TO PUMP RENIC P. VINCENT INVENTOR A T TORNE YUnited States Patent *Ofitice Patented Nov. 21, 1967 3,353,851 PNEUMATICCYLINDER FOR APPLYING TENSION T RISER PIPE Renic P. Vincent, Tulsa,Okla, assignor to Pan American Petroleum Corporation, Tulsa, Okla, acorporation of Delaware Filed Nov. 26, 1963, Ser. No. 325,855 4 Claims.(Cl. 285-302) ABSTRACT OF THE DISCLOSURE In rotary drilling from afloating drilling barge or similar support, it is found convenient toemploy a tubular conduit system frequently called a riser pipe assemblybetween the floating vessel and the solid ground below the water as ameans of confining the return of the drilling fluid from the mud line tothe floating vessel. This assembly is arranged with a flexible jointnear the bottom to permit horizontal displacement of the drill rig withrespect to the top of the well at the mud line, and a slip joint ortelescoping section, which provides for longitudinal changes in lengthof the assembly required due to vertical movement of the floatingvessel. If this slip joint is adjacent the floating vessel, as I prefer,there is a long section from the slip joint to the mud line which is acolumn under compression. Mechanically this is undesirable. Varioussystems have been proposed for placing this part of the riser pipeassembly under tension. The gist of my invention insists in applyinghydraulic (frequently pneumatic) pressure within the slip joint in sucha way that this pressure applies an upward force to the lower section ofthe riser pipe and a downward force to the top section of the riserpipe, to place the assembly in tension. Such hydraulic pressure isapplied in a predetermined amount, preferably from the floating vessel,through an at least semi-flexible hose to the variable volume cellformed between parts making up the slip joint.

This invention pertains to the art of marine drilling, and moreparticularly to a method for applying a stabilizing force to theso-called riser pipe used in such marine drilling operations in deepwater. Such drilling (usually to produce petroleum) is often carried outat the present time in deep water by using a floating drilling vesselcarrying a suitable derrick and other drilling equipment. This isanchored over the well location. At the sea floor some type of specialapparatus is cemented in place to define the upper end of the well. Inthe rotary system of drilling, which is customarily employed, theproblem then arises as to how to orient the drill string so that eachtime after replacement of a drill bit the string will re-enter theportion of the well already drilled, and so that during the drillingoperation the drilling fluid employed can be circulated between thedrilling vessel and the bottom of the well without substantialcontamination from the sea water surrounding the location. The drillingvessel itself, of course, is not completely stable in location even withthe best means of anchoring known. Due to action of the wind and ofwaves it rolls, pitches, and moves laterally. In addition, tidal actioncauses the drilling vessel to change its main elevation by quite anumber of feet.

One means of being sure that the mud returns will be uncontaminated andthat the drill string will retrace its path each time after the bit isreplaced (or during cementing operations) is to connect the apparatus atthe sea floor to the floating drilling vessel by what is called a riserpipe, which is an assembly making up a tubular conduit attached at thebottom to the sea floor apparatus and at the top to the drill rig on thefloating vessel. Such riser pipe must contain at least one flexiblejoint which is ordinarily provided near the lower end, and at least onetelescoping section near the upper end. This provides, therefore, forboth angular displacement of the drill rig axis relative to the hole atthe sea floor and also for changes in elevation between the vessel andthe sea floor.

In the first attempts to use riser pipes, difliculties and mechanicalfailures were encountered which ultimately were determined to take placedue to lack of elastic stability of the riser pipe.

The riser pipe, which is a heavy columnar member, may lose its elasticstability, if it is too long. It is also self-evident that the criticallength, i.e., the maximum length for which the column is stable whileself-supporting, depends on the pipe diameter and wall thickness. Asgreater and greater depths are considered, a riser pipe length isreached for which the column ceases to be selfsupporting, and to avoidits buckling, the upper termination of the column must be supportedthrough subjecting this termination to a certain amount of tension.

It is known to those skilled in the art that the critical length of ahollow column member, such as the riser pipe, depends also on thedifference of densities between the fluid inside and outside of themember. When the fluid becomes denser inside than outside, the criticallength becomes smaller, and more tension must be applied at the uppertermination to maintain the elastic stability.

When critical conditions are reached, the riser pipe will buckle even inabsence of any wave forces. Such forces may, however, bend the pipe toomuch and cause its failure, even when the length of the column is belowthe critical value. Subjecting the upper termination of the riser pipeto tension decreases the amount of bending and may prevent suchfailures.

This development will be described in connection with the appended threefigures. In these figures:

FIGURE 1 shows a typical floating drilling vessel supporting a riserpipe assembly suitably connected at the drilling base to vprotectiveequipment, and in which the telescoping section or slip joint has beenprovided with my invention;

FIGURE 2 shows a portion of the riser pipe assembly including in greaterdetail the slip joint shown in FIGURE 1. Part of the drawing in FIGURE 2is in cross section;

FIGURE '3 represents a mathematical relationship or graph used inconnection with this stabilizing method.

In FIGURE 1, the sea floor is represented diagrammatically at 11.Mounted on this is an assemblage of apparatus including a heavy plate 12with an upper portion 13, a sub 14, and a connection 15 to a blowoutpreventer 16, which is equipped with hydraulically-actuated rams, thehydraulic pressure being obtained from the drilling vessel (not shown)through suitable high pressure lines to the blowout preventer 16 (alsonot shown). Above this (with some other apparatus not important to thisinvention) may be and desirably is mounted a second blowout preventer 17also equipped for remote hydraulic actuation from the loading drillingvessel. Above this is a connection 18 to the start of the riser pipeassembly 50. This whole set of apparatus is shown diagrammatically.

It has already been mentioned that one function of the riser pipe is toprovide freedom of angular movement between the drilling vessel and thedrill string (at the point where the string passes through), forexample, at the plate 12. Accordingly, above a short sub at the bottomof the riser pipe 19 is mounted what is called a flex joint 28, wellknown in this art. This flex joint consists of a metal sleeve attachedat the bottom to member 53. The flex joint has been cut through atseveral different vertical levels around the circumference to make aplurality of mechanically interlinked sections with considerable angularmechanical freedom. Thus, the metal conduit can permit an angulardeflection of the upper pipe section 19 attached by connector 51 to theflex joint 20 through an angle of the order of 4 to 7 degrees in anydirection, but the sections of this pipe are held together, much thesame as interlocking pieces of a jigsaw puzzle.

This gives a required longitudinal strength but does not close off theriser pipe from the sea water outside the riser. Accordingly, the cutsections are covered with a reinforced rubber sleeve held tightlyagainst the inner flexible metal sleeve at the top and bottom bytwo-piece metal clamps. The rubber sleeve is ordinarily also coveredwith fabric, for example, a layer of tightly wrapped Manila rope, or thelike.

Above the top of the flex joint, the riser pipe 19 continues, preferablyup to the order of 30 to 40 feet below the surface of the water, thevarious joints of this conduit being connected together by clamps or thelike, as is well known in this art.

It is necessary to compensate for vertical changes in elevation of thedrilling vessel 52, which is held over the apparatus mounted above thesea floor 11, for example by anchoring or mooring lines 53. This isaccomplished by using what is called a slip joint which permits one partof the riser pipe 19 to telescope within another part 22 (see FIGURE 2).This slip joint ordinarily provides about feet of travel to permitVertical movement of the vessel. The slip joint is equipped with packing27 which tends to render the friction joints substantially fluid-tight.The upper conduit section 22 is connected through some sort of couplingmechanism such as a clamp 23 to the bottom of the drilling assembly 24which is mounted on the drilling vessel itself. The drill string, andthe various strings of casing, are run through the inside of the riserpipe.

It is apparent from this description that the apparatus described inFIGURES 1 and 2 includes a column extend: ing from the sea floor 11 upto the top of member 19 which is in compression, and a second memberconsisting of the upper conduit 22 and the apparatus above it, all ofwhich is in tension. It is well known that a column in compression maybuckle. One factor which afiects such buckling is the weight in fluidper unit length of the column, which is in compression. This weight w isequal to the weight per unit length of the riser pipe section 19 plusthe weight per unit length of the fluid inside the pipe, less the weightper unit length of an equivalent column of displaced fluid (i.e., seawater) outside the pipe.

, From this mathematical treatment it is possible to determine acritical height which is the maximum length of j the column incompression for which the system is stable.

This is given for the riser pipe by the expression where:

' L =critical weight I=moment of inertia of the pipe E=Youngs modulus ofpipe W=weight of column/unit length I where:

W ,=weight of pipe/ unit length W =density of fluid inside pipe W=density of fluid outside pipe For the derivation of Equation 1, see forinstance A Study of the Buckling of Rotary Drilling Strings, by ArthurLubinski, published in the 1950 vol. of API Drilling and ProductionPractice. For Equation 2, see for instance Buckling of Tubing in PumpingWells, Its Effects and Means for Controlling It, by Arthur Lubinski andK. A. Blenkarn, published in the Transactions of AIME (PetroleumBranch), 1957, vol. 210.

This Equation 1 does not take into consideration any lateral force suchas a wave force on the riser. If such wave forces were zero, the maximumlength, i.e., that length at which the pipe would just buckle would begiven by the equation. At that length, or at a greater length, the riserwould be expected to fail even in the absence of wave forces. To give anidea of this height, for ordinary 16-inch pipe weighing 62.6 pounds perfoot with a wall thickness of of an inch and with sea water in and out,the critical length is approximately 328 feet. However, if the riserpipe were conducting a mud with a density of 18 pounds per gallon, andon the assumption that the sea water density was 9 pounds per gallon,the corresponding critical length is reduced to 250 feet.

The presence of lateral forces, such as wave forces, causes the pipe tobend. From a practical standpoint it is found that such bending may beexcessive only when the actual length of the column under compression isfairly close to the critical length. It seems sufficiently safe to adoptin practice as a maximum length of the unsupported riser pipe a valueequal to about of the critical value L Thus, in the case justconsidered, one would conclude that a length of about 187 feet is themaximum that one should use for the section of the 16-inch riser pipe incompression.

If the length of the riser pipe is greater than the value considered,i.e., as the depth of water increases, one must in some fashion provideadditional support for the riser pipe. Various systems have beensuggested to accomplish this. Essentially they all involve in somefashion applying an upward force to the column otherwise undercompression, i.e., to place that colunm to a degree under tension. Usingthe first reference cited above, it may be shown that the amount ofdesired tension can be obtained on the following basis: First onedetermines the critical length L in accordance with Equation 1, givenabove. The ratio of the actual length L of the desired column up to thetop of member 19, divided by the critical length L is then determined.This value (L/L will be referred to as the factor 1. One then enters thegraph shown in FIGURE 3 to determine a corresponding quantity X.Finally, the necessary tension or total upward force which must beapplied is computed from the following formula:

where T is the tensile force to be applied at the upper extremity ofmember 25. It can be seen by reference to FIGURE 3 that if F=0'.75 thenX =0, which means that a length of riser pipe no longer than A of thecritical length requires no application of tension. For greater valuesof the length of the riser pipe, i.e., for deeper water, the methodexplained above gives the necessary value of tension which will give thesame degree of safety as that which will occur with the unsupportedriser pipe not longer than A of the critical length. Thus,

if the length of the riser pipe under compression in the example givenabove were 32.5. feet, the factor i would equal X =l.67, and T=22,000pounds.

Various methods have been suggested for applying tension to a riser pipeto avoid the difficulty of possible buckling or excessive bending. Forexample, the Rhodes et al. US. Patent 3,017,934 suggests the use ofbuoyant supports for the casing extending upwardly around the main riserpipe itself. It is also known to attach the upper part of the riser pipe19 directly to the vessel above with cables which pass through pulleysafiixed to the bottom of the drilling vessel down to weights, which thussupply tension to the lower portion 19 of the riser pipe. However, thereare real difiiculties in both of these attempts at solution of theproblem of stabilizing the riser pipe. When one employs buoyantsupports, the necessary consequence is a considerable increase of thearea exposed to wave forces, which results in a considerable increase intendency to bend the riser pipe assembly-the very defeet that thebuoyant chambers were attempting to prevent. If, on the other hand, oneemploys hanging weights connected through flexible cables and pulleys tothe upper part of the lower member of the riser pipe, increased waveaction is found on the weights themselves and there is a very seriousservicing problem involved on the pulleys, as well as the tensioningcables connecting the weights with the riser pipe.

I have found that it is possible to stabilize the riser pipe in thistype of marine drilling operations (that is, those involving drilling indeep water) which consists in applying tension to the riser pipe bymeans of fluid pressure applied to the telescoping section of the riserpipe. Reference to FIGURE 2 will illustrate this. Here the lower portionof the riser pipe 19 terminates in an upper end 25 while the upperportion of the riser pipe 22 terminates in a lower end 26. The twosections of the riser pipe 19 and 22 are designed with an appreciabledifference in diameter and each is provided with packing 27 so that theupper and lower telescoping portions of the riser pipe, together withthe ends or faces 30 and 31 thereof, form a variable volume cell 28. Theterm variable volume cell is used since it is apparent that upon motionof the telescoping sections the volume of the chamber 28 will changeaccordingly. A separate conduit 29 is connected to the variable volumecell, preferably just above the lower end 26 of the upper conduit 22.The conduit 29 is connected to an appropriate source of fluid pressure(not shown) through a quick disconnect joint 32 and conduit 33.

It is apparent from the description of the apparatus that when fluidpressure is applied through conduit 29 to the variable volume cell 28that an upward pressure is exerted on the lower face 30 of the end 25and similarly on the upper face 31 of lower end 26, thus applying anupward force on member 19 and a downward force on member 22 of equalmagnitude. This, therefore, is a means of applying the required tensionto the riser pipe assembly so that it is possible to maintain it stableunder all conditions. The drilling crew need simply be instructed tomaintain the force required above that given by Equation 3. The tensionis equal to the fluid pressure in cell 23 times the area of face 30. I,

It is apparent that in utilization of this method of applying tension tothe riser pipe that any fluid which can be conducted through conduit 29to the variable volume cell 28, may be employed. However, I find itparticularly advantageous to use a gas, for example, compressed air,compressed nitrogen, or the like, because of the fact that there isalmost continual motion between the sections 19 and 22 of the riserpipe, as the floating vessel responds to wave and wind action. If aliquid is used for applying pressure in the variable volume cell, suchtelescoping action of these two parts of the riser pipe cause anappreciable. difference in tension, increasing as the variable volumecell decreases in size and vice versa. However, it is desirable tomaintain this tension substantially constant, except when there is anincrease in the density of the drilling fluid. As is apparent uponreference to the equations given above, when the drilling fluid densityincreases there is need for an appropriate increase in the tensionapplied and, accordingly, the operators should increase the pressurewithin the variable volume cell 28. Now, if the volume of cell .28 isappreciable and if a gas is, used to apply pressure, within this cell,then minor changes in the length of this variable volume cell, inaccordance with Boyles law, will give only 'minor changes in pressure.In order to further minimize such pressure changes, it is desirablethough not strictly essential to provide a surge tank 34 at someconventional point connected in parallel with the pump (not shown) toconduit 33. (The volume of such tank 34 should preferably be at least aslarge as the maximum volume in cell 28.)

It is not to be assumed, however, that liquids cannot be employed for,in fact, they will operate well, the only difference being that therewill be a larger component of varying tension under ordinary operatingconditions, since liquids have less compressibility than gases. If aliquid is used, it is necessary to provide surge tank 34 as discussedabove, with at least the major part of its volume being occupied at alltimes by a gas.

The method of stabilizing which has been described, may be used when thelower portion of riser pipe 19 is shorter than the 75 percent ofcritical length. However, there is no particular reason for suchstabilization under that conditon. However, as soon as L has appreciablyexceeded the 75 percent of critical length of riser pipe, as illustratedby the plot in FIGURE 3, tension is needed in the riser pipe tostabilize it and keep it from buckling. In this case the invention willfurnish all of the stability needed.

I claim:

1. Means for stabilizing a riser pipe in deep water marine drillingoperations by maintaining said riser pipe in tension, said riser pipebeing connected between a floating vessel and the marine floo-r andincluding at least one telescoping section near the bottom of saidvessel, said section comprising inner and outer conduit sectionsslidingly sealed to each other at the ends of said section therebyforming a fluid tight variable volume cell therebetween, each said endof said sections carrying a sealing means thereon which forms said cellbetween said inner and outer conduit, said variable volume cell being soconstructed that the axial distance between the cell ends of saidsection can increase, thereby placing the riser pipe in tension, saidtension being mantained by supplying only fluid under pressure to thevariable volume cell formed within said telescoping section by saidconduit sections, whereby the tendency of said riser pipe to columnbuckling and bending during operations is minimized.

2. The process according to claim 1 in which said fluid pressure isapplied pneumatically by meansv of a 3. The process according to claim 1in which said fluid pressure is maintained substantially constant aslong as the drilling fluid density remains substantially constant and isincreased upon increase of said drilling fluid density.

4. Equipment for marine drilling operations from a floating vesselcomprising a riser pipe assembly attached at one end to said floatingvessel and at the other end to the marine floor and containingtherebetween a telescoping section including an inner and an outerconduit section comprising part of the confining walls of said riserpipe assembly, said inner conduit section having an outer diameter lessthan the inner diameter of the outer conduit section and having anenlarged head portion, said outer pipe section provided with a radiallyinwardly directed portion, the periphery of said head portion and saidradially inwardly directed portion provided with seal means thereon,said head portion, radially inwardly directed portion and the walls ofthe inner and outer conduit sections contained therebetween, forming asubstantially fluid-tight variable volume cell in said section, saidouter pipe section provided with conduit connecting means in the wallthereof in the area of said variable volume cell, a source of fiuidpressure, and a conduit connected to said source and to said means onsaid variable volume cell, by means of which fluid pressure in said cellprovides tensile force between the ends of said assembly to decreasetendency for column buckling 0r bending in said assembly.

References Cited UNITED STATES PATENTS Kamrnerer 175-5 X Bellinger 28518Rhodes et al 175220 X Kofahl 166-665 Buoy 287-20 X Pollard et a1166-66.5 X Lacy 16666.5

CARL W. TOMLIN, Primary Examiner.

DAVE W. AROLA, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,353,851 November 21, 1967 Renic P. Vincent It is hereby certified thaterror appears in the above numbered patent requiring correction and thatthe said Letters Patent should read as corrected below Column 3, line 7,for "28" read 20 line 70, for "weight" read height column 6, line 17,for "conventionaj read convenient Signed and sealed this 17th day ofDecember 1968.

(SEAlJ Attest:

EDWARD J. BRENNER Commissioner of Patents Edward M. Fletcher, Jr.

Attesting Officer

1. MEANS FOR STABILIZING A RISER PIPE IN DEEP WATER MARINE DRILLINGOPERATIONS BY MAINTAINING SAID RISER PIPE IN TENSION, SAID RISER PIPEBEING CONNECTED BETWEEN A FLOATING VESSEL AND THE MARINE FLOOR ANDINCLUDING AT LEAST ONE TELESCOPING SECTION NEAR THE BOTTOM OF SAIDVESSEL, SAID SECTION COMPRISING INNER AND OUTER CONDUIT SECTIONS SLIDINGSEALED TO EACH OTHER AT THE ENDS OF SAID SECTION THEREBY FORMING A FLUIDTIGHT VARIABLE VOLUME CELL THEREBETWEEN, EACH SAID END OF SAID SECTIONSCARRYING A SEALING MEANS THEREON WHICH FORMS SAID CELL BETWEEN SAIDINNER AND OUTER CONDUIT, SAID VARIABLE VOLUME CELL BEING SO CONSTRUCTEDTHAT THE AXIAL DISTANCE BETWEEN THE CELL ENDS OF SAID SECTION CANINCREASE, THEREBY PLACING THE RISER PIPE IN TENSION, SAID TENSION BEINGMANTAINED BY SUPPLYING ONLY FLUID UNDER PRESSURE TO THE VARIABLE VOLUMECELL FORMED WITHIN SAID TELESCOPING SECTION BY SAID CONDUIT SECTIONS,WHEREBY THE TENDENCY OF SAID RISER PIPE TO COLUMN BUCKING AND BENDINGOPERATIONS IS MINIMIZED.